Method for producing an aluminum piston for an internal combustion engine

ABSTRACT

A method for producing an aluminum piston for an internal combustion engine, which piston is designed as a round skirt piston. In the region of the radially outer end faces of the piston pin bosses the piston skirt has skirt recesses extending in the direction of the piston axis. In order to be able, when using the piston, to produce skirt recesses that have any shape a piston blank is produced in a forging process using a forging die, which piston blank has inner faces that are complementary to the outer shape of the piston having skirt recesses and not-recessed regions of the piston skirt, and the piston blank can then be further machined to produce the aluminum piston.

The invention relates to a method for the production of an aluminum piston for an internal combustion engine, which is configured as a round-skirt piston, having a piston crown, having a ring belt and having a piston skirt that follows the ring belt in the direction facing away from the piston crown, which skirt has two pin bosses that lie opposite one another, whereby the piston skirt has skirt recesses that extend in the direction of the piston axis, in the region of the radially outer face surfaces of the pin bosses.

A round-skirt-piston of the type stated initially is known from the European patent application EP 0 167 791 A2, having skirt recesses that extend over the entire axial length of the piston skirt in the region of the pin bosses, so that these regions do not have any contact with the inner cylinder wall after installation of the piston into the engine. These recesses are formed into the piston skirt by means of a lathe, whereby the piston is clamped into the lathe eccentrically in its longitudinal direction, in order to be able to lathe the skirt recesses into the piston skirt. It is disadvantageous, in this connection, that using the lathing methods known from the state of the art, it is only possible to produce skirt recesses whose depth decreases continuously toward the edge of the skirt recesses, so that they have an oval shape in cross-section.

It is therefore the task of the present invention to propose a method for the production of a piston having skirt recesses, which recesses can be shaped in any desired manner.

This task is accomplished in that the piston is produced using the forging method, using a forging die, the inner surface of which is configured to be complementary to the outer shape of the piston, particularly the shape of the piston skirt, the skirt recesses, and the non-recessed regions of the piston skirt. In this connection, the inner surface of the forging die can be given any desired shape, so that any desired outer shape of the piston can also be produced with it.

Practical embodiments of the invention are the object of the dependent claims.

Some exemplary embodiments of the invention will be explained in the following, using the drawings. These show:

FIG. 1 a piston in a side view; whereby the piston skirt is recessed in the region of the face surfaces of the pin bosses,

FIG. 2 a section through the piston along the line AA in FIG. 1, and

FIG. 3 A, B two embodiments of the transitions between the recessed and the non-recessed skirt regions.

FIG. 1 shows a piston 1 for an internal combustion engine, which is configured as a round skirt piston, consists of aluminum, and is produced using the forging method. The piston 1 has a piston crown 2, a top land 3, a ring belt 4, and a piston skirt 5 having pin bosses 6, 6′ and pin bores 7, 7′.

In the region of the face surfaces 8, 8′ of the pin bosses 6, 6′, the piston skirt 5 has skirt recesses 9, 9′, in other words the piston skirt 5 is recessed, in these regions, by 0.5 mm to 2 mm in the radial direction, and these regions 9, 9′ thus have a lesser distance from the piston axis 10, by 0.5 to 2 mm, than the non-recessed regions 11, 11′ of the piston skirt 5.

In this connection, the transition regions 12, 12′ between the skirt recesses 9, 9′ and the non-recessed regions 11, 11′ of the piston skirt 5 can lie parallel to one another, as in the present exemplary embodiment according to FIG. 1, or (not shown in FIG. 1) they can narrow conically toward one another in the direction of the piston crown 2 or in the direction of the lower end 15 of the piston skirt 5, facing away from the piston crown. Furthermore, the skirt recesses 9, 9′ can be shaped like the mantle surfaces of a circular cylinder. However, they can also have an oval shape, viewed in cross-section perpendicular to the piston axis 10, whereby the axis 13 of the pin bore 7 (FIG. 2) is the small axis of ovality.

In the present exemplary embodiment, the axial height 14 of the skirt recess 9, 9′ reaches over the entire axial length of the piston skirt 5, in other words from the ring belt 4 all the way to the lower end 15 of the piston skirt 5, facing away from the piston crown. However, the height 14 of the skirt recess 9, 9′ can be freely selected here.

In FIG. 2, a section through the piston 1 along the line AA in FIG. 1, it is shown that the circumferential expanse of the skirt recesses 9, 9′ and of the non-recessed regions 11, 11′ of the piston skirt 5 can be defined using the angle α between a straight line 16 that stands perpendicular on the axis 13 of the pin bore 7 and furthermore perpendicular on the piston axis 10, and a straight line 17 that connects the piston axis 10 with the transition region 12′. In the present exemplary embodiment, the angle α has a value of 32°. However, it can amount to between 5° and 85°.

FIG. 3 shows two embodiments A and B of the transition regions 12 a and 12 b between the skirt recesses 9 and the non-recessed regions 11 of the piston skirt 5. The transition region 12 a according to FIG. 3A has an edge 18 and 19 that reaches over the entire height 14 of the skirt recess 9, as compared with the regions 9 and 11, in each instance. The transition region 12 b according to FIG. 3B connects the two regions 9 and 11 continuously and without edges.

The circumferential expanse of the transition regions 12 a and 12 b can be defined by way of the angle β, β′ between straight lines 20, 21 that connect the edges of the regions 11 and 9 with the piston axis 10 (not shown in FIG. 3) and stand perpendicular on the piston axis 10. This angle β, β′ can amount to between 5° and 20°.

Forming of the skirt recesses 9, 9′ takes place, within the scope of forging technology production of the piston 1, using a forging die configured to be complementary to the piston skirt 5. In this way, first a piston blank 1 having the skirt recesses 9 and 9′ and the non-recessed regions 11, 11′ of the piston skirt 5 is produced.

Production of a piston blank using the forging method is known from the state of the art (for example from DE 10 2005 041 000) and will not be explained in any detail. Here it should only be noted that in the case of a forging apparatus that consists of a mandrel and a forging die, the mandrel serves for forming the piston interior, and the forging die serves for forming the outer piston surface. In this connection, the forging die has a shape complementary to the outer piston surface according to FIG. 2, whereby the forging die does not, however, have any formed parts that correspond to the grooves of the ring belt 4 and the pin bores 7, 7′.

During forging of the piston blank, the non-recessed regions 11, 11′ of the piston skirt 5 and the skirt recesses 9, 9′ are therefore formed. The advantage of the forging method according to the invention, for production of a piston blank, consists in that any desired shapes of the skirt recesses 9, 9′ and of the transition regions 12, 12′ can be produced in simple manner. For example, skirt recesses having a shape that is round in cross-section, but also recesses having a shape that is oval in cross-section can be produced in this way. Furthermore, the transition regions 12 a and 12 b, respectively, can be structured between the skirt recesses 9, 9′ and the non-recessed regions 11, 11′ of the piston skirt 5, in such a manner that they lie parallel to the piston axis 10 or narrow conically toward one another either in the direction of the piston crown 2 or in the direction of the lower end 15 of the piston skirt 5.

Subsequent to the forging process, the pin bores 7, 7′ are produced by means of a drilling machine, and the grooves of the ring belt 4 are produced using a lathe.

In this connection, the reduction of the surfaces 11, 11′ of the piston skirt 5 that stand in contact with the inner cylinder surface, by means of the skirt recesses 9, 9′, has the advantage of improving the slide properties of the piston. Furthermore, a round-skirt piston having skirt recesses offers longer pin bores 7, 7′ in the direction of the pin axis 13, as compared with a box piston having face surfaces of the pin bosses that are set back relative to the ring belt, and thus a greater contact surface for the piston pin, so that round-skirt pistons can be put under stress with greater combustion pressures.

Furthermore, the method according to the invention has the advantage that the skirt recesses can be shaped in any desired manner. Seen in the direction of the piston axis, the transition regions between the skirt recesses and the non-recessed regions of the piston skirt can have any desired shape, and can be configured in wave shape, for example.

REFERENCE SYMBOL LIST

-   α, β, β′ angles -   1 piston -   2 piston crown -   3 top land -   4 ring belt -   5 piston skirt -   6, 6′ pin boss -   7, 7′ pin bore -   8, 8′ face surface of the pin boss 6, 6′ -   9, 9′ skirt recess -   10 piston axis -   11, 11′ non-recessed region of the piston skirt 5 -   12, 12′ transition region between the skirt recess 9 and the region     11 -   12 a, 12 b transition region between the skirt recess 9 and the     region 11 -   13 axis of the pin bore 7 -   14 height of the skirt recess 9 -   15 lower end of the piston skirt 5 -   16 connection straight line -   17 connection straight line -   18, 19 edge between the transition region 12 a and the skirt recess     9 or the non-recessed region 11 of the piston skirt 5, respectively -   20, 21 straight line 

1. Method for the production of an aluminum piston (1) for an internal combustion engine, which is configured as a round-skirt piston, having a piston crown (2), having a ring belt (4) and having a piston skirt (5) that follows the ring belt (4) in the direction facing away from the piston crown, which skirt has two pin bosses (6, 6′) that lie opposite one another, wherein the piston skirt (5) has skirt recesses (9, 9′) that extend in the direction of the piston axis (10), in the region of the radially outer face surfaces (8, 8′) of the pin bosses (6, 6′), comprising the following method steps: production of a piston blank using the forging method, using a mandrel for forming the piston interior and a forging die for forming the outer shape of the piston (1), wherein the outer shape of the piston skirt (5), with the skirt recesses (9, 9′) and the non-recessed regions (11, 11′) of the piston skirt (5), which is complementary to the inner surface of the forging die, is formed in, final machining of the piston blank for the production of the piston (1), using a drilling machine for the production of the pin bores (7, 7′) and using a lathe for production of the grooves of the ring belt (4).
 2. Method according to claim 1, wherein the skirt recesses (9, 9′) are produced with a depth between 0.5 mm and 2 mm.
 3. Method according to claim 2, wherein transition regions (12 a) are formed in between the skirt recesses (9, 9′) and the non-recessed regions (11, 11′) of the piston skirt (5), which regions have an edge (18, 19) that reaches over the entire height (14) of the skirt recess (9, 9′), in each instance, as compared with the skirt recesses (9, 9′) and as compared with the non-recessed regions (11, 11′), and have a flat surface between the edges (18, 19).
 4. Method according to claim 2, wherein transition regions (12 b) are formed in between the skirt recesses (9, 9′) and the non-recessed regions (11, 11′) of the piston skirt (5), which regions connect the skirt recesses (9, 9′) with the non-recessed regions (11, 11′) of the piston skirt (5), continuously and without edges.
 5. Method according to claim 3, wherein during forging of the piston blank, the transition regions (12 a, 12 b) are structured in such a manner that they come to lie parallel to the piston axis (10).
 6. Method according to claim 3, wherein during forging of the piston blank, the transition regions (12 a, 12 b) are structured in such a manner that they narrow conically toward one another in the direction of the piston crown (2).
 7. Method according to claim 3, wherein during forging of the piston blank, the transition regions (12 a, 12 b) are structured in such a manner that they narrow conically toward one another in the direction of the end (15) of the piston skirt (5) that faces away from the piston crown. 